This invention relates generally to aluminothermic reduction (ATR) reaction mixtures, and more particularly to an ATR mixture which produces only a slag phase and is ideal for use in providing refractory linings in pipes and hollow articles.
ATR techniques have long been known for welding and repairing large metal articles. In these practices, an exothermic reduction reaction mixture, such as a stoichiometric blend of powdered metallic aluminum and iron oxide is ignited. The mixture then reacts rapidly and exothermically wherein the metallic fuel reduces the metal oxide to produce a molten metal phase and a molten slag phase. For ATR mixtures, containing aluminum and iron oxide, the molten phases produced are iron and alumina. By suitably confining or directing the molten reaction products, the molten metal phase can be deposited where desired to weld or repair large metal objects. The slag phase, which floats over the heavier metal phase, can be broken away and discarded.
More recently, ATR techniques have been utilized in processes wherein the slag phase is the desired product. For example, U.S. Pat. No. 4,150,182 teaches a process for providing a refractory liner within tubes, pipe and other hollow cylindrical articles. In this process, an ATR mixture is reacted within the tube or other cylindrical article, and pursuant to one embodiment, the article is rotated on its horizontal axis so that the molten metal phase is first deposited on the interior walls and then the slag phase is deposited thereover. This then provides a corrosion-resistant, abrasion-resistant ceramic liner within the article which is useful for many industrial applications.
In contrast to the prior art processes for providing refractory linings in pipes, cylinders, etc., the patented process described above is quick, inexpensive, requires little capital expenditure and requires no preconditioning of the pipe such as cleaning, pickling, furnace firing, etc. The patented process does, however, have some drawbacks which stem primarily from the fact that the ATR reaction produces an unnecessary metal phase as well as the desired slag phase. Specifically, the ATR process will normally deposit a thin layer of reduced metal phase under the refractory lining. Although this is not normally harmful, there are situations where the deposited metal and the metal pipe are not particularly compatible. This also tends to restrict the process to the lining of metal pipes. In addition, the effervescent nature of the ATR reaction causes the reaction products to spatter and, therefore, the desired refractory lining usually contains small particles of the reduced metal phase. In some applications, this metal could contaminate the fluid passing through the lined pipe.
In another embodiment of the above-mentioned U.S. Pat. No. 4,150,182, the pipe or other hollow cylindrical article is filled with an ATR mixture and disposed in a stationary vertical position. The ATR mixture is then ignited at the exposed upper end. The ATR reaction will progress downward through the article, producing the molten metal and slag phases. As the reaction progresses downward within the hollow article, the molten phases also move downward, whereupon a layer of the slag phase is deposited on the walls of the cooler hollow article. The metal phase does not normally deposit itself on the inner wall of the article in this embodiment but accumulates, increasing in volume as the reaction progresses. When exceptionally long pipes are being coated pursuant to this embodiment, the depth of the molten metal phase may become so great and the upper portion thereof so removed from the hot reaction zone that it will solidify, bridging across the opening through the pipe thereby plugging the pipe or other article. In very long sections of pipe, several such bridges may be formed. Accordingly, here again the unnecessary metal phase causes problems.